N2H4-H2O Enabled Umpolung Cyclization of o-Nitro Chalcones for the Construction of Quinoline N-Oxides

Base-Promoted and Copper(I)-Catalyzed Tandem Cyclization-C(sp2)-N Coupling of Vinyl Malononitriles with Ortho-Nitrochalcones: Access to Acridones and their Fused Derivatives

An efficient Cs2CO3-promoted and copper(I)-catalyzed double cyclization of ortho-nitrochalcones with vinyl malononitriles for the de novo access to a variety of tri- and tetra-substituted acridones and their fused derivatives with a value-added CN group has been developed for the first time. This one-pot operation proceeds through a Michael-cyclization-aromatization, followed by regioselective ipso-amination via nucleophilic aromatic substitution (SNAr) reaction, resulting in two C═C bonds and a C-N bond for acridone ring synthesis. This economic strategy based on 100% carbon atoms ensures the successive formation of two rings in a one-pot operation, good to high yields, a wide range of substrates, and good tolerance of functionalities. In addition, acridones were converted into several value-added acridones and acridines, highlighting the synthetic versatility and usefulness of the prepared acridone derivatives.

Harnessing the Reactivity of Nitroarene Radical Anions to Create Quinoline N-Oxides by Electrochemical Reductive Cyclization

Electrochemical reduction of 2-allyl-substituted nitroarenes using a simple, undivided electrochemical cell with non-precious electrodes to generate nitroarene radical anions was developed. The nitroarene radical anion intermediates participate in 1,5-hydrogen atom transfer reactions to construct quinoline N-oxides bearing aryl-, heteroaryl-, alkenyl-, benzyl-, sulfonyl-, or carboxyl groups.

Copper-Catalyzed Cascade Cyclization of 2-Nitrochalcones with NH-Heterocycles

We developed a method for allowing cascade cyclization of 2-nitrochalcones with pyrazoles, imidazole, and indazole in the presence of CuI catalyst, DBU base, and THF solvent. The conditions were tolerant of an array of useful functionalities including ester, nitro, cyano, halogen groups. A mechanistic consideration was also provided, as H2O2 was presumably a byproduct. Our method appears to be a rare example to directly prepare C3-heterocyclic unprotected indoles.

Bi(OTf)3-Catalyzed Cascade Cycloaddition-Decarbonylation-N-Deoxygenative Aromatization between 2-Arylisatogens and Styrenes: Unveiling a Synthesis of 2,4-Diarylquinolines

An unprecedented Bi(OTf)3-catalyzed reaction between 2-arylisatogens and vinylarenes to yield 2,4-diarylquinolines is reported. The transformation occurred via a BiIII-coordination to the embedded anionic nitrone oxygen, which induced a regiospecific stepwise formal [4 + 2]-cycloaddition with vinylarenes to yield a strained tricyclic intermediate, which subsequently extruded gaseous carbon monoxide. N-Deoxygenative aromatization ensued to produce 2,4-diarylquinolines. The protocol was applicable to a variety of substrates to produce the quinolines in up to an 89% yield.

Base-Catalyzed Cascade Cyclization of 2-Nitrochalcones and Isocyanides to Access Pyrano[3,4-b]indol-1(9H)-one Frameworks

An unexpected cascade reaction of 2-nitrochalcones with isocyanoacetates has been reported for the efficient synthesis of indole carboxylic esters and pyranoindoles. The conversion was achieved by KOH-catalyzed cyclization and elimination of the nitro group with final decarbonylation-aromatization. The method was used to synthesize a series of potentially biologically active indole derivatives (49 examples) in 67-85% yields under transition-metal-free catalytic conditions.

Synthesis of Aromatic N-Oxides Using Electrochemically Generated Peroxodicarbonate

Electrochemically generated green platform oxidizers like peroxodicarbonate (PODIC) constitute a game-changing technology in terms of sustainable chemistry while serving as an alternative counterreaction in the electrochemical hydrogen evolution. Peroxodicarbonate avoids the storage and shipping of concentrated hydrogen peroxide solution. We herein disclose an efficient method for the N-oxidation of quinolines, pyridines, and complex tertiary amines. The use of phenoyloxy succinimide (POSI) is the decisive factor for obtaining N-oxides (28 examples) in isolated yields of up to 98%.

Synthesis of 2-carboxyaniline-substituted maleimides from 2'-nitrochalcones

A practical, one-pot approach to 3-anilino-4-(het)arylmaleimides by simple heating of aqueous DMSO solution of 2'-nitrochalcones with potassium cyanide in the presence of formic acid has been developed. This new reaction provides effective access to a variety of β-substituted α-aminomaleimides which have recently become a subject of growing interest as small, easily modified and environmentally responsive fluorescent probes.

Direct Electrochemical Synthesis of 2,3-Disubstituted Quinoline N-oxides by Cathodic Reduction of Nitro Arenes

The use of electric current in synthetic organic chemistry offers a sustainable tool for the selective reductive synthesis of quinoline N-oxides starting from easily accessible nitro compounds. The reported method employs mild and reagent-free conditions, a simple undivided cell, and constant current electrolysis set-up which provides conversion with a high atom economy. The synthesis of 30 differently substituted quinoline N-oxides was successfully performed in up to 90 % yield. Using CV studies, the mechanism of the selective formation of the quinoline N-oxides was elucidated. The technical relevance of the described reaction could be shown in a 50-fold scale-up reaction.

Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

Reduction of the stable nitro group is a fundamental and widely used transformation for the construction of complex and functionalized heterocyclic architectures. The unfolding of the reactivity of the nitro group in the 2-nitrostyrene moiety not only triggers the formation of carbon-nitrogen bonds, but also offers the opportunity for annulation and heteroannulation, thereby providing a cascade process for the synthesis of highly conjugated natural and unnatural molecules. In this review, we comprehensively discuss the use of 2-nitrostyrene motifs in the synthesis of various N-heterocycles. We offer readers an overview of the synthetic achievements achieved to date, highlighting their important features, reactivities, and mechanisms.

Recent advances on synthesis and biological activities of C-17 aza-heterocycle derived steroids

Steroids modification for improving their biological activities is one of the most efficient and fruitful methods to develop novel medicines. Steroids with aza-heterocycles attaching to the C-17 owing various biological activities have received great attentions and some of the compounds are developed successfully as drugs. In this review, the research of the syntheses and biological activities of steroids bearing various aza-heterocycles published in the last 8 years is assembled, and some important structure-activity relationships (SARs) of active compounds are presented. According to the analysis of the literatures and our experiences in this field, the potential of aza-heterocyclic steroids as medicinal drugs is proposed.

Cu(I)-Catalyzed and Base-Promoted [5 + 2 + 1] Cascade Cyclization of 2-Nitrochalcones with Aliphatic Primary Amines to 5H-Pyrimido[5,4-b]indole Frameworks

An unprecedented [5 + 2 + 1] cascade cyclization to the preparation of 5H-pyrimido[5,4-b]indole derivatives has been disclosed. The novel protocol of 2-nitrochalcones reacted with aliphatic primary amines catalyzed by CuI and promoted by Cs₂CO₃, which underwent a critical intermediate 2,3-disubstituted indole, providing structurally diverse 5H-pyrimido[5,4-b]indoles in generally high yields (77-90%) and broad substrate scopes (34 examples).

Scaffold hopping by net photochemical carbon deletion of azaarenes

Discovery chemists routinely identify purpose-tailored molecules through an iterative structural optimization approach, but the preparation of each successive candidate in a compound series can rarely be conducted in a manner matching their thought process. This is because many of the necessary chemical transformations required to modify compound cores in a straightforward fashion are not applicable in complex contexts. We report a method that addresses one facet of this problem by allowing chemists to hop directly between chemically distinct heteroaromatic scaffolds. Specifically, we show that selective photolysis of quinoline N-oxides with 390-nanometer light followed by acid-promoted rearrangement affords N-acylindoles while showing broad compatibility with medicinally relevant functionality. Applications to late-stage skeletal modification of compounds of pharmaceutical interest and more complex transformations involving serial single-atom changes are demonstrated.